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Old 12-23-2012, 04:22 PM   #1
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Reuse existing intercom wires for low voltage power source?


I'm removing intercoms that are located throughout all rooms of my house. I plan to setup some Android tablets at each wall location, and would like to use the old intercom wiring to run a low voltage power supply so I can have USB port charging stations at each location.

The central intercom unit was in the kitchen, so I assume I can install some type of in-wall transformer to run the 5v 1.5A needed for USB to each room. If I were running it to just one location I think I'd know what to do, but I'm not sure exactly sure how to wire it up to go to 8 rooms and whether I can do that with a single transformer or if other electronic parts would be required?

Any help is greatly appreciated..

Thanks -Gary.

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Old 12-23-2012, 04:59 PM   #2
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Reuse existing intercom wires for low voltage power source?


That old intercom most likely used 18g or smaller wire.

1.5A through that small of a wire any kind of distance is most likely going to have way too much voltage drop.

Look at the wire...if it's 16g or larger, you might be ok...but if it's 18g or smaller....I think you will have issues with voltage.

Worse case....try it....go to the outlet that is the longest distance from your power source....and do a temp hook up....measure the voltage....with the unit plugged in, if you above 4.75v...I guess you will be ok....maybe.....if the lines are daisy chained....then I see more issues....

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Old 12-23-2012, 05:06 PM   #3
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Reuse existing intercom wires for low voltage power source?


Would be easier to just use the charger that comes with them. As for the android tablets, are you using them for some type of connected home setup? As for your idea, works great on paper, but not in real life when you actually do it.
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Old 12-23-2012, 05:35 PM   #4
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Reuse existing intercom wires for low voltage power source?


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Originally Posted by ddawg16 View Post
Look at the wire...if it's 16g or larger, you might be ok...but if it's 18g or smaller....I think you will have issues with voltage.
Yes, unfortunately it looks like 18g I don't think they are daisy chained though, I'd like to give it a try..

Assuming the distance is not too great, for multiple locations would I simply connect every wire pair from each room location together to the same +/- terminals on a 5v 1.5A transformer? Would that work? or would I need some other power/wiring configuration for this?
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Old 12-23-2012, 05:44 PM   #5
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Reuse existing intercom wires for low voltage power source?


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Originally Posted by gcristof View Post
Yes, unfortunately it looks like 18g I don't think they are daisy chained though, I'd like to give it a try..

Assuming the distance is not too great, for multiple locations would I simply connect every wire pair from each room location together to the same +/- terminals on a 5v 1.5A transformer? Would that work? or would I need some other power/wiring configuration for this?
Well.....if every USB port needs 5Vdc @ 1.5A.....then multiply the number of drops you have by 1.5A....and that will tell you what size power supply you need.

Lets also clarify something....the USBis 5V DC.....not AC....so a simple transformer will not work. You will need a 5Vdc power supply.
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Old 12-23-2012, 05:46 PM   #6
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Reuse existing intercom wires for low voltage power source?


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Would be easier to just use the charger that comes with them. As for the android tablets, are you using them for some type of connected home setup? As for your idea, works great on paper, but not in real life when you actually do it.
Unfortunately most of the wall locations where I intend to place them don't have any outlets to plug in the charger nearby.

The tablets are mostly used for streaming music, browser, email, etc., just starting to use them to also control some zwave devices.
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Old 12-23-2012, 06:18 PM   #7
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Reuse existing intercom wires for low voltage power source?


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Well.....if every USB port needs 5Vdc @ 1.5A.....then multiply the number of drops you have by 1.5A....and that will tell you what size power supply you need.
Gotcha, so for 8 drops I would need 5v 12A? Would increasing the voltage a little bit possibly help overcome a distance issue? (I think all of the runs are close to the same length).

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Originally Posted by ddawg16 View Post
Lets also clarify something....the USBis 5V DC.....not AC....so a simple transformer will not work. You will need a 5Vdc power supply.
Yes, that much I know.. I haven't searched yet but thought I could find an in-wall power supply with the correct DC out. Alternatively I suppose I could use a 5v 12A wall wart and wire up to that..
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Old 12-23-2012, 06:25 PM   #8
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Reuse existing intercom wires for low voltage power source?


5v @ 12A is a pretty good size power supply......in my world, your looking a $100 or more for one that size....and thats going cheap.

I really think this is not going to be a good way to go....for one thing...if the power supply fails....you loose all your ports....not just one.
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Old 12-23-2012, 07:02 PM   #9
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Reuse existing intercom wires for low voltage power source?


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Originally Posted by gcristof View Post
Gotcha, so for 8 drops I would need 5v 12A? Would increasing the voltage a little bit possibly help overcome a distance issue? (I think all of the runs are close to the same length).


Yes, that much I know.. I haven't searched yet but thought I could find an in-wall power supply with the correct DC out. Alternatively I suppose I could use a 5v 12A wall wart and wire up to that..
Be careful with wall-warts. Often they're very cheaply made, so the 5V DC might have considerable ripple, and they're poorly regulated, meaning that 5V under load might rise to 8 or 9V with no load, which might fry some ICs in the USB circuit of the device.
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Old 12-23-2012, 11:58 PM   #10
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Reuse existing intercom wires for low voltage power source?


Would something like this be worth a try for only $18?
http://www.ebay.com/itm/5V-12A-60W-S...item2571515071
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Old 12-24-2012, 12:32 AM   #11
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Reuse existing intercom wires for low voltage power source?


Since the wire is so small, and because USB requires tightly regulated voltage, your best bet would be to run higher DC voltage and install a 5V converter at each location. If the lines all go to a central point (so that each run only handles one tablet, not all of them chained together) then you could run 12VDC and use USB car chargers at each location for regulation.
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Old 12-24-2012, 12:33 AM   #12
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Reuse existing intercom wires for low voltage power source?


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Originally Posted by gcristof View Post
Gotcha, so for 8 drops I would need 5v 12A? Would increasing the voltage a little bit possibly help overcome a distance issue? (I think all of the runs are close to the same length)...
No, do not increase the voltage to overcome distance or voltage drop issues.

In the case of chargers, the current they output gets smaller as the battery gets more and more charged. In turn the voltage at the far end (the device with the battery) will increase to approach the voltage at the supply end because the volts lost in the wires equals the amperes being drawn at the moment times the resistance of the wires (Ohm's Law). You and the battery and the device don't want this to happen.
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Old 12-24-2012, 02:22 AM   #13
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Reuse existing intercom wires for low voltage power source?


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No, do not increase the voltage to overcome distance or voltage drop issues.

In the case of chargers, the current they output gets smaller as the battery gets more and more charged. In turn the voltage at the far end (the device with the battery) will increase to approach the voltage at the supply end because the volts lost in the wires equals the amperes being drawn at the moment times the resistance of the wires (Ohm's Law). You and the battery and the device don't want this to happen.
The USB standard is a tightly regulated 5VDC supply, with digitally "negotiated" current capacity. Devices that charge from USB have the charging circuitry in the device, not in the USB host. So supplying more than 5V will not overcharge the battery. However, it will damage the device since the expected voltage is 4.75 -5.25V. Anything outside that range is outside the device ratings and could damage it. 5.5V could really be pushing it, and 6V would likely be real bad. The bottom line is that the USB voltage regulation must be done at the device, not at the other end of a long #18 cable.
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Old 12-24-2012, 08:02 AM   #14
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Reuse existing intercom wires for low voltage power source?


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Originally Posted by gcristof View Post
Unfortunately most of the wall locations where I intend to place them don't have any outlets to plug in the charger nearby.

The tablets are mostly used for streaming music, browser, email, etc., just starting to use them to also control some zwave devices.
Is there really that much of a demand in your household for that many Android devices?

It is easier to pull up a outlet for charging, than hacking together something that will not work, unless you know what you are doing. What are the tablets and docking stations that you plan on using, so others like myself can further help on this dream.
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Old 12-24-2012, 08:46 AM   #15
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Reuse existing intercom wires for low voltage power source?


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Originally Posted by mpoulton View Post
Since the wire is so small, and because USB requires tightly regulated voltage, your best bet would be to run higher DC voltage and install a 5V converter at each location. If the lines all go to a central point (so that each run only handles one tablet, not all of them chained together) then you could run 12VDC and use USB car chargers at each location for regulation.
This sounds like a good solution, that's what a USB car charger is made to do, and the one's I've used haven't fried any of my devices (not yet anyway)..
Q. So if I run 12v 12A everywhere and terminate at USB car adapters, would this still work correctly when loads are not present at each location (if several of the devices are unplugged)?

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Last edited by gcristof; 12-24-2012 at 08:54 AM.
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